Recently, development of electronic paper has enthusiastically been performed.
The electronic paper, which features the use of a display apparatus as paper, calls for characteristics which are different from a related-art display apparatus such as a CRT (cathode ray tube) or a liquid crystal display. For example, it calls for the characteristics that it is a reflective display apparatus, it has a high white reflectance and a high contrast ratio, it can perform a highly fine display, the display is provided with a memory effect, it may be driven with a low voltage, it is thin and light, it is inexpensive, etc. Of these, there is a particularly high demand for a white reflectance and a contrast ratio which are equivalent to paper as the characteristics related to a quality of display.
On the other hand, for the display apparatuses used for the electronic paper, schemes have been proposed such as a scheme using a reflective liquid crystal; a scheme using electrophoresis; and a scheme using phoretic motion of toner. However, in any of the above-described schemes, it is very difficult to perform multi-color displaying while maintaining the white reflectance and contrast ratio. In order to perform multi-color displaying in general, while a color filter is provided, the reflectance decreases since the color filter itself absorbs light. Moreover, as the color filter divides one pixel into three: red (R), green (G), and blue (B), the reflectance of the display apparatus further decreases. When the white reflectance and contrast ratio significantly decrease, viewability becomes very poor, making it difficult to use as electronic paper.
On the other hand, a promising technique for realizing a reflective display apparatus, not providing the color filter as described above, is a scheme using electro-chromic phenomenon (electro-chromism).
The electro-chromism is a phenomenon in which applying a voltage induces reversible oxidation and deoxidation reactions, so that color changes reversibly. An electro-chromic display element is a display apparatus which utilizes color developing/reducing (below called color developing and reducing) of an electro-chromic compound which induces the electro-chromism phenomenon.
The electro-chromic display element is a reflective display apparatus, has a memory effect, and may be driven with a low voltage, so that extensive research and development from material development to device design is being carried out on it as a prospective candidate of a display apparatus technique for use in electronic paper. Moreover, the electro-chromic display element is expected to be a multi-color display apparatus since it may develop various colors according to the structure of the electro-chromic compound.
There are a number of known examples for the multi-color display apparatus which utilize such electro-chromic display elements.
For example, Patent document 1 discloses a multi-color display apparatus using electro-chromic compounds, wherein fine particles of multiple types of electro-chromic compounds are laminated. In the above document, an example is disclosed of a multi-color display apparatus, wherein multiple electro-chromic compounds are laminated. The electro-chromic compounds are polymer compounds with multiple functional groups having different voltages at which color developing is manifested.
Moreover, in Patent document 2, a display apparatus is disclosed, wherein electro-chromic layers are formed in multiple layers on an electrode and multiple colors are developed using differences in voltage and current values necessary for color developing. Patent document 2 discloses an example of a multi-color display apparatus having a display layer formed by laminating or mixing multiple electro-chromic compounds which develop different colors and which have different electric charge amounts required for color developing and threshold voltages at which color is developed.
Moreover, Patent document 3 discloses an electro-chromic display element, wherein multiple display electrodes and corresponding multiple electro-chromic layers are laminated on a display substrate. The electro-chromic display element, which has a high white color level, may develop multiple colors individually in a simple method and provide a color display at a wide color range.